In medical diagnosis and treatment of a subject, it is often necessary to assess one or more physiological characteristics, particularly respiratory characteristics. A key respiratory characteristic is respiratory air volume (or tidal volume). Respiratory air volume and other respiratory characteristics are also useful to assess athletic performance, for example, by aiding in detection of changes in physiological state and/or performance characteristics.
Monitoring physiological and performance parameters of a subject can be important in planning and evaluating athletic training and activity. A subject may exercise or otherwise engage in athletic activity for a variety of reasons, including, for example, maintaining or achieving a level of fitness, to prepare for or engage in competition, and for enjoyment. The subject may have a training program tailored to his or her fitness level and designed to help him or her progress toward a fitness or exercise goal. Physiological and performance parameters of a subject can provide useful information about the subject's progression in a training program, or about the athletic performance of the subject. In order to accurately appraise the subject's fitness level or progress toward a goal, it may be useful to determine, monitor, and record various physiological or performance parameters, and related contextual information.
In the past, various methods and systems utilizing heart rate have been introduced to approximate effort and physiological stress during exercise. However, convenient, practicable, and comfortable means of measuring pulmonary ventilation in non-laboratory conditions have been scarce. While of good value, heart rate can only give an approximation as to the true physiological state of an athlete or medical patient, as it can be confounded by external factors including, for example, sleep levels, caffeine, depressants, beta blockers, stress levels, hydration status, temperature, etc. Furthermore, accurate use of heart rate to gauge physiological performance requires knowledge of the amount of blood flowing to the muscles, which in turn requires knowledge of the instantaneous stroke volume of the heart as well as the rate of pumping. These parameters can be difficult to determine while a subject is engaging in a physical activity.
Various conventional methods and systems have been employed to measure (or determine) tidal volume. One method comprises having the patient or subject breathe into a mouthpiece connected to a flow rate measuring device. Flow rate is then integrated to provide air volume change.
As is well known in the art, there are several drawbacks and disadvantages associated with employing a mouthpiece. A significant drawback associated with a mouthpiece and nose-clip measuring device is that the noted items cause changes in the monitored subject's respiratory pattern (i.e., rate and volume). Tidal volume determinations based on a mouthpiece and nose-clip are, thus, often inaccurate.
A mouthpiece is difficult to use for monitoring athletic performance as we as for long term monitoring, especially for ill, sleeping or anesthetized subjects. It is uncomfortable for the subject, tends to restrict breathing, and is generally inconvenient for the physician or technician to use. Monitoring respiratory characteristics using a mouthpiece is particularly impractical in the athletic performance monitoring context. During athletic activities, the mouthpiece interferes with the athlete's performance. The processing and collection accessories necessary to monitor the breathing patterns captured by the mouthpiece add further bulk to such devices. These systems also typically require an on-duty technician to set up and operate, further complicating their use.
Another conventional device for determining tidal volume includes respiration monitors. Illustrative are the systems disclosed in U.S. Pat. No. 3,831,586, issued Aug. 27, 1974, and U.S. Pat. No. 4,033,332, issued Jul. 5, 1977, each of which is incorporated by reference herein in its entirety.
Although the noted systems eliminate most of the disadvantages associated with a mouthpiece, the systems do not, in general, provide an accurate measurement of tidal volume. Further, the systems are typically only used to signal an attendant when a subject's breathing activity changes sharply or stops.
A further means for determining tidal volume is to measure the change in size (or displacement) of the rib cage and abdomen, as it is well known that lung volume is a function of these two parameters. A number of systems and devices have been employed to measure the change in size (i.e., Δ circumference) of the rib cage and abdomen, including mercury in rubber strain gauges, pneumobelts, respiratory inductive plethysmograph (RIP) belts and magnetometers. See, D. L. Wade, “Movements of the Thoracic Cage and Diaphragm in Respiration”, J. Physiol., pp. 124-193 (1954), Mead, et al. “Pulmonary Ventilation Measured from Body Surface Movements”, Science, pp. 196, 1383-1384 (1967).
RIP belts are a common means employed to measure changes in the cross-sectional areas of the rib cage and abdomen. RIP belts comprise conductive loops of wire that are coiled and sewed into an elastic belt. As the coil stretches and contracts in response to changes in a subject's chest cavity size, a magnetic field generated by the wire changes. The output voltage of an RIP belt is generally linearly related to changes in the expanded length of the belt and, thus, changes in the enclosed cross-sectional area.
In practice, measuring changes in the cross-sectional areas of the abdomen can increase the accuracy of RIP belt systems. To measure changes in the cross-sectional areas of the rib cage and abdomen, one belt is thus typically secured around the mid-thorax and a second belt is placed around the mid-abdomen.
RIP belts can also be embedded in a garment, such as a shirt or vest, and appropriately positioned therein to measure rib cage and abdominal displacements, and other anatomical and physiological parameters, such as jugular venous pulse, respiration-related intra-plural pressure changes, etc. Illustrative is the VivoMetrics, Inc. LifeShirt® disclosed in U.S. Pat. No. 6,551,252, issued Apr. 22, 2003 and U.S. Pat. No. 6,341,504, issued Jan. 29, 2002, each of which is incorporated by reference herein in its entirety.
However, there are some drawbacks to many RIP belt systems. For example, RIP belts are expensive in terms of material construction and in terms of the electrical and computing power required to operate them. In addition, the coils are generally large and tight on the chest and therefore can be cumbersome and uncomfortable for the athlete.
Other technologies have been developed in an attempt to monitor respiratory characteristics of a subject while avoiding the drawbacks of RIP belt systems. These technologies generally work on a strain gauge principle and are often textile based. However, such technologies suffer significantly from motion interference that, by and large, renders them useless in athletic training applications where motion is necessarily at a relatively high level.
In an attempt to rectify the drawbacks of the RIP belt and strain gauge systems, various magnetometer systems have been recently developed to measure displacements of the rib cage and abdomen. Respiratory magnetometer systems typically comprise one or more tuned pairs of air-core magnetometers or electromagnetic coils. Other types of magnetometers sensitive to changes in distance therebetween can also be used. One magnetometer is adapted to transmit a specific high frequency AC magnetic field and the other magnetometer is adapted to receive the field. The paired magnetometers are responsive to changes in a spaced distance therebetween; the changes being reflected in changes in the strength of the magnetic field.
To measure changes in (or displacement of) the anteroposterior diameter of the rib cage, a first magnetometer is typically placed over the sternum at the level of the 4th intercostal space and the second magnetometer is placed over the spine at the same level. Using additional magnetometers can increase the accuracy of the magnetometer system. For example, to measure changes in the anteroposterior diameter of the abdomen, a third magnetometer can be placed on the abdomen at the level of the umbilicus and a fourth magnetometer can be placed over the spine at the same level.
Over the operational range of distances, the output voltage is linearly related to the distance between two magnetometers provided that the axes of the magnetometers remain substantially parallel to each other. As rotation of the axes can change the voltage, the magnetometers are typically secured to the subject's skin in a parallel fashion and rotation due to the motion of underlying soft tissue is minimized.
Various methods, algorithms and mathematical models have been employed with the aforementioned systems to determine tidal volume and other respiratory characteristics. In practice, “two-degrees-of-freedom” models are typically employed to determine tidal volume from RIP belt derived rib cage and abdominal displacements.
The “two-degrees-of-freedom” models are premised on the inter-related movements by and between the thoracic cavity and the anterior and lateral walls of the rib cage and the abdomen, i.e., since the first rib and adjacent structures of the neck are relatively immobile, the moveable components of the thoracic cavity are taken to be the anterior and lateral walls of the rib cage and the abdomen. Changes in volume of the thoracic cavity will then be reflected by displacements of the rib cage and abdomen.
As is well known in the art, displacement (i.e., movement) of the rib cage can be directly assessed with an RIP belt. Diaphragm displacement cannot, however, be measured directly. But, since the abdominal contents are essentially incompressible, caudal motion of the diaphragm relative to the pelvis and the volume it displaces is reflected by outward movement of the anterolateral abdominal wall.
The “two-degrees-of-freedom” model embraced by many in the field holds that tidal volume (VT) is equal to the sum of the volume displacements of the rib cage and abdomen, i.e.:VT=αRC+βAb  Eq. 1                where RC and Ab represent linear displacements of the rib cage and abdomen, respectively; and α and β represent volume-motion coefficients.        
The accuracy of the “two-degrees-of-freedom” model and, hence, methods employing same to determine volume-motion coefficients of the rib cage and abdomen, is limited by virtue of changes in spinal flexion that can accompany changes in posture. Indeed, it has been found that VT can be over or under-estimated by as much as 50% of the vital capacity with spinal flexion and extension. See, McCool, et al., “Estimates of Ventilation From Body Surface Measurements in Unrestrained Subjects”, J. Appl. Physiol., vol. 61, pp. 1114-1119 (1986) and Paek, et al., “Postural Effects on Measurements of Tidal Volume From Body Surface Displacements”, J. Appl. Physiol., vol. 68, pp. 2482-2487 (1990).
There are two major causes that contribute to the noted error and, hence, limitation. A first contributing cause of the error is due to the substantial displacement of the summed rib cage and abdomen signals that occurs with isovolume spinal flexion and extension or pelvic rotation.
The second contributing cause of the error is due to posturally-induced changes in volume-motion coefficients. With isovolume spinal flexion, the rib cage comes down with respect to the pelvis and the axial dimension of the anterior abdominal wall becomes smaller. Therefore, less abdominal cavity is bordered by the anterior abdominal wall
With a smaller anterior abdominal wall surface to displace, a given volume displacement of the abdominal compartment would be accompanied by a greater outward displacement of the anterior abdominal wall. The abdominal volume-motion coefficient would accordingly be reduced.
It has, however, been found that the addition of a measure of the axial motion of the chest wall, e.g., changes in the distance between the xiphoid and the pubic symphysis (Xi), provides a third degree of freedom, which, when employed to determine tidal volume (VT) can reduce the posture related error associated with the “two-degrees-of-freedom” model to within 15% of that measured by spirometry. See, Paek, et al., “Postural Effects on Measurements of Tidal Volume From Body Surface Displacements”, J. Appl. Physiol., vol. 68, pp. 2482-2487 (1990); and Smith, et al. “Three Degree of Freedom Description of Movement of the Human Chest Wall”, J. Appl. Physiol., Vol. 60, pp. 928-934 (1986).
Several magnetometer systems are thus adapted to additionally measure the displacement of the chest wall. Illustrative are the magnetometer systems disclosed in co-pending U.S. patent application Ser. No. 12/231,692, filed Sep. 5, 2008, which is incorporated by reference herein in its entirety.
Various methods, algorithms and models are similarly employed with the magnetometer systems to determine tidal volume (Vr) and other respiratory characteristics based on measured displacements of the rib cage, abdomen and chest wall. The model embraced by many in the field is set forth in Equation 2 below:VT=α(ΔRC)+β(Δab)+γ(ΔXi)  Eq. 2                where:        ΔRC) represents the linear displacement of the rib cage;        ΔAb represents the linear displacement of the abdomen;        ΔXi represents axial displacement of the chest wall;        α represents a rib cage volume-motion coefficient;        β represents an abdominal volume-motion coefficient; and        γ represents a chest wall volume-motion coefficient.        
There are, however, similarly several drawbacks and disadvantages associated with the noted “three-degrees-of-freedom” model. A major drawback is that posture related errors in tidal volume determinations are highly probable when a subject is involved in freely moving postural tasks, e.g., bending, wherein spinal flexion and/or extension is exhibited.
The most pronounced effect of spinal flexion is on the abdominal volume-motion coefficient (β). With bending, β decreases as the xiphi-umbilical distance decreases.
Various approaches and models have thus been developed to address the noted dependency and, hence, enhance the accuracy of tidal volume (VT) determinations. In co-pending U.S. patent application Ser. No. 12/231,692, a modified “three-degrees-of-freedom” model is employed to address the dependence of β on the xiphi-umbilical distance, i.e.,VT=α(ΔRC)+βu+εXi0×(ΔAb)+γ(ΔXi)  Eq. 3                where:        ΔRC represents the linear displacement of the rib cage;        ΔAb represents the linear displacement of the abdomen;        ΔXi represents the change in the xiphi-umbilical distance from an upright position;        α represents a rib cage volume-motion coefficient;        β represents an abdominal volume-motion coefficient;        βu represents the value of the abdominal volume-motion coefficient (β) in the upright position;        ε represents the linear slope of the relationship of β as a function of the xiphi-umbilical distance Xi:        (βu+εXi) represents the corrected abdominal volume-motion coefficient; and        γ represents a xiphi-umbilical volume-motion coefficient.        
The “three-degrees-of-freedom” model reflected in Equation 3 above and the associated magnetometer systems and methods disclosed in co-pending U.S. patent application Ser. No. 12/231,692 have been found to reduce the posture related error(s) in tidal volume (VT) and other respiratory characteristic determinations. There are, however, several issues with the disclosed magnetometer systems and methods. One issue is that the magnetometer systems require complex calibration algorithms and associated techniques to accurately determine tidal volume (VT) and other respiratory characteristics. A further issue, which is discussed in detail herein, is that the chest wall and respiratory data provided by the disclosed systems (and associated methods) is limited and, hence, limits the scope of respiratory characteristics and activity determined therefrom.